The washing of solid particles with a washing liquid has been frequently carried out in the production of organic and inorganic chemical products. Recently, soils contaminated with harmful substances such as dioxin are washed with a washing liquid such as water for regeneration.
The washing of solid particles basically includes a step for transferring impurities in the solid particles into a washing liquid and a step for separating the solid particles from the washing liquid. In the former stage, the impurities are removed from the solid particles by dissolution into the washing liquid or by dispersion into the washing liquid after divided into finer particles. A tank equipped with a stirrer has been frequently used to enhance the removing efficiency and to increase the transferring speed of the impurities into the washing liquid. The impurities can be almost completely transferred into the washing liquid by modifying the structure of the washing tank and controlling the residence time of the solid particles therein.
In the latter stage, the solid particles are separated by discarding the supernatant after allowing a slurry to stand or by a solid-liquid separation method such as filtration and centrifugal precipitation. However, a some amount of the washing liquid is generally retained in the solid particles separated by these separation methods. The washing liquid itself retained in the solid particles is removed by drying, but impurities in the washing liquid remains in the solid particles without being evaporated to result in an insufficient removal of the impurities.
Therefore, to sufficiently remove the impurities by washing the solid particles, it is required to reduce the amount of the washing liquid that accompanies the solid particles during the separation procedure. To enhance the effect of washing the solid particles, there has been used a separator in which a washing liquid containing impurities is removed by sprinkling a fresh washing liquid on the separated particles in the separator. However, such a separator involves problems that the structure is complicated and a sufficient washing effect is not obtained when the size of solid particles are small. Another approach for enhancing the effect of washing the solid particles is a washing method using a combination of a number of washing tanks and separators. Since the centrifugal separators and rotary filter separators which are frequently used in industrial processes are expensive, the method using a number of these apparatuses increases installation costs. In addition, there is proposed a method of sufficiently washing solid particles using a number of liquid cyclones (JP 5-140044 A). The cyclone itself is an inexpensive separator having a simple structure. However, a number of pumps are required to recycle the washing liquid, this making the overall system complicated. Therefore, the proposed method is not necessarily inexpensive. Further, the proposed method is not applicable to solid particles that are easily crushed because the particles are crushed in pumps and cyclones. Therefore, it has been demanded to develop a method of sufficiently washing solid particles by using an apparatus with simpler structure.
Another problem to be solved upon washing solid particles is to reduce the amount of a used washing liquid to be discharged as the waste. In the washing of crystals for the production of chemical products and the washing of contaminated soil as described above, the direct discharge of the used washing liquid causes environmental pollution. To avoid this problem, the used washing liquid should be discharged after decomposing the impurities or making the impurities harmless by chemical or biochemical treatments. It is advantageous for the decomposition or the treatment of making harmless that the amount of the waste liquid is smaller and the impurities are more concentrated therein, because the size of apparatus can be reduced and the energy required can be saved. In case of the removal of harmful substances such as dioxin which must be removed to an extremely low concentration, the waste liquid is difficult to be made harmless efficiently with low costs by the conventional methods, because the amount of the waste liquid is large and the concentration of impurities in the waste liquid is low. For example, a waste water of the same amount as that of soil being washed must be made harmless (Example 1 of JP 2001-113261 A), or a washing water three times the amount of soil to be washed is required (Examples of JP 2001-47027).